There are two main ways to achieve air and vapor sampling. On the one hand, the active sampling, which requires physical drawing of a volume of air through a collection medium by means of a suction pump. On the other hand, the passive sampling, in which the collection is achieved thanks to an adsorbent material. The present invention relates to the field of passive air sampling, also called passive diffusive sampling.
Passive diffusive sampling relies on the diffusion of analytes through a diffusive surface onto an adsorbent. After sampling, the analytes are chemically desorbed by solvent extraction or thermally desorbed and analyzed. Passive sampling does not involve the use of heavy and encumbering pumping systems, is not impacted by power disruptions, does not require extensive supervision, is quiet, non-flammable and does not represent an explosion hazard. It can be performed by anyone, anywhere and at a very low cost. Moreover, it is not susceptible to sample breakthrough, a common problem associated with active sampling performed with an air pump.
The passive sampling allows the analyte to be collected owing to the concentration gradient between the bulk ambient air concentration and a retention medium (i.e. solid adsorbent, liquid or gel absorbent). That means that the up-take or sampling rate of a device will depend on the compound (diffusivity), area and length of diffusion (i.e. design parameters, short of diffusion or permeation media), and adsorbent characteristics (adsorption isotherm). Parameters like adsorption isotherm, adsorption capacity, sampling rate and variables like concentration level and sampling time are correlated, in such a way, that for instance, high concentrations of analyte in ambient air will saturate the adsorbent and decrease its sampling rate; in a similar way, a high sampling rate will more quickly saturate the adsorbent, although would allow higher sensitivity and shorter sampling times. Longer sampling times may cause similar saturation effects.
In practice, the sampling rate that applies to a diffusive sampler is valid for a particular analyte-adsorbent combination over a certain range of concentration and sampling time.
The uptake rate of a diffusive sampler depends on its design: length and resistance along the diffusion path from ambient air to the adsorbent bed. Most of the diffusive samplers have been designed for a specific application, which provide sampling rates that are sometimes very low or very high to prevent the saturation of the adsorbent or its use for longer times or vice versa.
Some diffusive samplers contain diffusion membranes and even if they are re-useable, they can easily deteriorate with time, becoming dirty with graphitized adsorbent or by exposure to ambient conditions. The possibility of cleaning is limited as the membranes cannot be separated from the sampler.
The material of the diffusive sampler is sometimes a handicap for the measurement of certain compounds, which can contaminate the sample or act as a competitor for adsorption.
The transport, storage and interruption of the sampling for a certain period of time (for instance for interrupted cumulative personal sampling) is difficult, to handle as the sample may easily become contaminated.
This explains the diversity of designs, adsorbents and variations of diffusion samplers on the market. Each diffusive sampler, in principle, has been designed for a particular application; i.e. short or long term sampling time, different environment (workplaces, ambient air, indoor air, and personal exposure), different compounds, analytical techniques and levels of concentration. Documents U.S. Pat. No. 4,350,037, EP 2 063 248, WO 2008/015031, US 2003/0215958 describe several different kinds of diffusive samplers. Nevertheless, some of these samples are used in different fields, under different sampling conditions; which may affect an optimized performance of the diffusive sampler, where its operational conditions are limited by design to a certain range.